1. Field of the Invention
This invention relates generally to the preparation and fabrication of prepreg tapes and ribbons for automated tow placement and relates specifically to the preparation of polyimide resin/carbon fiber/boron fiber unidirectional solvent-free tape and ribbon having encapsulated boron fibers evenly distributed throughout the tape or ribbon.
2. Description of the Related Art
Automated tow placement (ATP) is a process where composite ribbons or tapes are robotically managed and continually fed onto a tool or part surface and adhered by the application of heat and pressure. ATP is particularly sensitive to the quality of the composite ribbon when working with low-flow matrix materials. The simultaneous assembly of adjacent ribbons (typically 4 to 34) or wide tape offers significant advances in the lay-up of composite materials. However, ribbons or tapes made from low-flow matrix materials typically lack a cross-sectional dimensional integrity, and more importantly, a standard rectangular cross-section. These structural defects complicate the ATP process and frequently render poor results. Although ribbons are bonded to their vertical neighbor (directly below) satisfactorily, the failure to make quality parts is generally attributed to the poor bonding of adjacent ribbons to each other and the quality of tape being used. Low-flow thermoplastic parts made by using slit prepreg tapes are typically unconsolidated and exhibit excessive porosity and void content.
Ideally, tapes used in the ATP process should be fully consolidated. Consolidation means the voids in a composite material have been eliminated during melt-processing. One method of accomplishing consolidation is pultrusion. This technique requires full ingestion of the unconsolidated composite material within an enclosed die with an exit area less than the inlet area. Pultrusion has a slow production rate.
Another method of accomplishing some consolidation is disclosed by Sandusky in U.S. Pat. Nos. 5,395,47. Sandusky discloses an apparatus and method for providing a consolidated, unidirectional, continuous, fiber-reinforced composite ribbon. The apparatus includes a pre-melting chamber, a stationary bar assembly, and a loaded, cooled nip-roller apparatus. Examples given by Sandusky discuss the manufacture of ribbons having a width of 0.250 in. and three inches.
Sandusky employs a number of techniques which make the fabrication of a uniform, fully consolidated three-inch wide tape difficult. First, the shaping means has a fixed gap between its rollers which leads to the fabrication of material with a fixed cross-section. Without the adaptiveness of a variable cross-section, a precise uniform-shaped tape or ribbon will be produced only if enough composite material is fed through the forming device to fill the fixed opening. During periods when the powder content of the fiber tow drops below the desired amount, the shape of the tape or ribbon becomes irregular and jagged, which facilitates the generation of voids.
Second, Sandusky lacks a collimation device for maintaining the orientation of each individual fiber tow bundle. Such a device is essential in the manufacture of a uniform product with respect to thickness. If the individual tows are out of alignment they can overlap and create a tape which is not a consistent thickness across its width.
Third, the lack of a dedicated drive component for the processing line is a critical problem. The tows need to maintain a zero speed differential across the width of the tape during processing. If the individual tows are moving at differing velocities, the tape will not combine and consolidate. Accordingly, a process for producing a fully-consolidated, uniform tape is desired. One example of a dry powder impregnation method is disclosed in U.S. patent application Ser. No. 09/185,142, filed Nov. 3, 1998, entitled Method and Apparatus to Fabricate a Fully-Consolidated Fiber-Reinforced Tape from Polymer Powder Preimpregnated Fiber Tow Bundles for Automated Tow Placement (Belvin et al.), the disclosure of which is herein incorporated by reference. U.S. patent application Ser. No. 09/185,142 discloses the manufacture of a three-inch wide product from powder pre-impregnated fiber-tow bundles.
Typically, carbon fiber composites can provide excellent mechanical properties. However, in certain applications carbon fiber alone does not provide adequate compressive strength. A solution to this problem is the hybridization of carbon fiber composites with boron reinforcing fibers.
Current technology for making boron/carbon prepreg and composites from polyimides includes utilizing solution coated prepreg with residual solvent. Boron fibers are calendered onto xe2x80x9cwetxe2x80x9d prepreg to make a hybrid boron/carbon fiber prepreg. This technique only pushes the large diameter boron fibers partially into the xe2x80x9cwetxe2x80x9d prepreg. The resultant prepreg contains boron fibers that are essentially sitting on the prepreg surface and are not fully encapsulated within resin. This material also requires solvent removal during cure, a process which is labor intensive and costly.
The need for a technique to process high temperature polyimides into composites without a solvent is apparent. The hazards and expense of solvent removal and recovery are critical to this composite technology. Developing a dry polyimide tape will allow for ATP by a robot and will significantly reduce the cost of processing composite materials into parts. A process which utilizes significantly less or no solvent and results in a higher quality intermediary and end product is essential to the use of these polyimide composites in large quantities.
Accordingly, an object of the present invention is to manufacture a hybrid boron reinforced polymer matrix composition.
Another object is to manufacture a hybrid boron reinforced polymer matrix composition of consistent thickness across its width.
Another object of the invention is to manufacture a hybrid boron reinforced polymer matrix composition with minimal voids therein.
Still another object of the present invention is to manufacture a hybrid boron reinforced polymer matrix composition having geometric accuracy regardless of changes in the powder content along the length of the fiber tow bundles.
The foregoing and additional are attained by providing a method of producing a consolidated, hybrid boron-carbon fiber-reinforced composite tape comprising the steps for dispensing at least one linear array of boron fibers; sandwiching the dispensed linear array of the boron fibers with a plurality of powder pre-impregnated fiber tow bundles, heating the sandwiched linear array of the boron fibers to a specific processing temperature which melts the polymeric matrix of the tow bundles; spreading the heated tow bundles across an impregnation bar assembly to encapsulate the linear array of boron fibers with the heated tow bundles and to shape the heated tow bundles to an initial width, wherein the encapsulated boron fibers are dispersed in a substantially uniform manner; pulling the shaped tow bundles using a drive mechanism, thereby enabling the polymeric matrix of the shaped tow bundles to consolidate fully into the hybrid boron-reinforced composite tape; and taking-up the consolidated hybrid boron-reinforced composite tape.
The present invention further includes an apparatus for producing a consolidated, hybrid boron-reinforced composite tape comprising a dispensing means for supplying at least one linear array of the boron fibers; an applying means for positioning a plurality of powder pre-impregnated fiber tow bundles along a length of the dispensed linear array of the boron fibers; a processing component for heating the positioned tow bundles and linear array of the boron fibers to a specific processing temperature which melts the polymeric matrix of the tow bundles and disperses boron fibers into the polymeric matrix, the processing component having an entrance and an exit, the processing component including an impregnation bar assembly positioned near the exit of the processing component for wetting-out the filament array of the heated tow bundles with boron fibers and for spreading the heated tow bundles with boron fibers to an initial width; a variable dimension forming nip means, positioned in operable relationship to the processing component, for shaping the heated precursor tape into a predetermined width, the variable dimension forming nip means being actively cooled and capable of imparting a selected pressure, wherein the hybrid boron reinforced polymer matrix composite tape is formed; a driving means, positioned in operable relationship to the variable dimension forming nip means, for pulling the shaped tow bundles with boron fibers, the driving means maintaining a constant speed across the width of the shaped tow bundles with boron fibers, thereby enabling the polymeric matrix of the shaped tow bundles with boron fibers to consolidate fully into the consolidated, boron-fiber-reinforced composite tape; and means for taking-up the boron-fiber-reinforced composite tape.
Additionally, the present invention includes a hybrid boron reinforced polymer matrix composition produced from the process comprising the steps of providing powder pre-impregnated fiber tow bundles; applying at least one linear array of boron fibers along the length of the powder pre-impregnated fiber tow bundles; heating the powder pre-impregnated fiber tow bundles and applied boron fibers to a specific processing temperature which melts the polymeric matrix of the heated powder pre-impregnated fiber tow bundles; and encapsulating the applied boron fibers with the powder pre-impregnated fiber tow bundles, wherein a hybrid boron reinforced polymer matrix composition is formed.
The method for manufacturing the hybrid boron reinforced polymer matrix composition begins with the powder pre-impregnated fiber tow bundles being mounted onto a pay-out creel for delivery. Concurrently, a linear array of boron fibers is dispersed and applied to the powder pre-impregnated fiber tow bundles. The spools of tow bundles are individually tensioned at this point to facilitate the alignment of the filaments of each bundle. This tensioning also aids in the spreading of the bundles in the processing component. The tow bundles are then fed through a collimation device to maintain the alignment of the individual tow bundles during processing. The collimation device ensures consistent thickness across the width of the processed material. If the alignment changes, a tape or ribbon will have an irregular shape and will not be a placeable grade tape, i.e., acceptable for use with the ATP process.
This alignment also facilitates the forming of the molten pre-preg or polymeric matrix into a precise shape and dimension. The bundles then proceed through the processing component. The processing component comprises two parts: an oven or furnace and an impregnation bar assembly. The oven is heated to a specific processing temperature for each individual polymeric matrix depending on the powdered resin of the tow bundles. Preferably, when processing requires a high temperature to melt the polymeric matrix material, an inert gas such as nitrogen is used as a process medium inside the oven to induce melting without oxidation. While still inside the oven, the tow bundles are pulled through the impregnation bar assembly. This assembly encapsulates or wets-out the filaments of the tow bundles and spreads the tow bundles to an initial width and shape. The tension which is created back at the pay-out creel is instrumental in this spreading process, with greater tension further assisting the spreading of the fiber tow.
Upon exiting the process component, the molten tow bundles with boron are fed through the variable dimension forming nip means having two rollers. The variable dimension forming nip means cools the molten tow bundles and shapes them into a precise, predetermined width. Preferably, the invention uses nitrogen as the cooling medium. Additionally, because the variable dimension forming nip means does not have a defined gap between the two rollers, the rollers allow for changes in powder content along the fiber tow bundles during processing by varying the cross-section along the length of the composite tape. Powder content can vary along the length of the towpreg as much as xc2x18% depending on the type process used to coat the clean tow bundles.
The next component is the self-contained driving means. The self-contained driving means pulls the total number of bundles and boron fibers needed to fabricate the tape through the process. The driving means maintains the speed of the process and removes any speed differential between the individual tow bundles. This constant speed in turn eliminates a shearing force which would create gaps and splits in the finalized tape. Thus, the driving means allows the resin content of the bundles to flow together. As a result, the method produces a fully-consolidated hybrid boron composite tape, which is spooled by a motorized take-up system.